Digital communication has revolutionized telephone networks. One particular aspect, Voice over Internet Protocol (VoIP), has provided increased efficiency and quality for real-time voice communications. VoIP utilizes protocols to initiate and control call parameters. For example, Session Initiation Protocol (SIP) is an application-layer protocol that is used to set up VoIP phone calls. SIP INVITE messages may indicate contact information to initiate a call to a called user. Other types of SIP messages may be used to query capabilities or match codecs for a VoIP call.
As telephone technology has advanced, telephonic surveillance techniques have also changed. Rather than placing a monitoring device on a particular phone handset, a gateway may monitor VoIP call messages at the core of the packet network. Existing VoIP surveillance techniques rely on intercepting SIP messages that identify a calling party or a caller party. For example, an SIP INVITE message may include the IP address of a called user.